Make sure bringForward returns a denotation valid in current period

There was a scenario in the IDE where this was not the case: If the owner of
a denotation was deleted in the new run, the old symbol gets an updated validity.
Then, a call to bringForward of a member of that invalid owner will lookup
the symbol in its owner, yielding itself. But the validity still was the previous period.

Author: odersky

compiler/src/dotty/tools/dotc/core/Denotations.scala

@@ -791,7 +791,7 @@ object Denotations {
           lazy val staleOK = ctx.acceptStale(symd)
           if (ctx.settings.YupdateStale.value && symd.owner.is(Package) || staleOK) {
             val newd = symd.owner.info.decls.lookup(symd.name)
-            if (newd.exists) return newd
+            if (newd.exists) return (newd.denot: SingleDenotation).updateValidity()
             else if (staleOK) return updateValidity()
           }
         case _ =>